1. Field of the Invention
The present invention relates to a vehicular radio wave receiver and an information displaying apparatus with radio wave receiver.
2. Description of Related Art
Recently, various radio wave receivers are mounted on vehicles. A keyless entry system is used as a kind of the receivers in vehicles. The keyless entry system has a receiver and a transmitter. The receiver is mounted on a vehicle, and the transmitter is included in a key carried by a driver. The transmitter modulates signals that include an identification (ID) code and an operation code, and transmits the modulated signals to the receiver. When the receiver receives the modulated signals, it demodulates the signals and determines whether the demodulated ID code corresponds to an ID code of the receiver. Then, when the received ID code corresponds to the ID code of the receiver, the receiver sends control signals to electrical control units (ECU) in the vehicle so that the doors are opened or closed, and an engine is started. The keyless entry system generally uses weak signals in 300 MHz frequency band. The receiver is installed in an appropriate place so that the receiver can have a gain as high as possible.
In JP-A-H-08-216735, an instrument panel has a receiver and an antenna as well as a control circuit for controlling the instrument panel. In such a receiver, because the antenna is disposed close to a window of the vehicle, the receiver is less likely to be prevented from transmitting and receiving by a metallic body of the vehicle wherever the driver is.
It is desirable that a length of an element of the antenna corresponds to λ/4. Here, “λ” is a wavelength. However, because the instrument panel has a rectangular shape and is limited to a certain size, the instrument panel may not have an enough space for the antenna even when the element of the antenna is disposed in a lateral direction. On the other hand, if the element of the antenna is shorter than λ/4, sensitivity of the antenna becomes low.
A dielectric antenna, which measures approximately 20 millimeters (mm) by 5 mm by 5 mm, is known as a downsized antenna. If the dielectric antenna is used, the dielectric antenna can be easily installed inside the instrument panel because of its size.
A vehicular navigation system is used in the vehicle. In the vehicular navigation system, a control circuit of the navigation system is connected to a Global Positioning System (GPS) receiver. The GPS receiver receives GPS signals from GPS satellites, and sends them to the control circuit to calculate a position of the vehicle. In such a navigation system, the GPS receiver is separated from the control circuit. If the dielectric antenna is used for the GPS receiver, it is thinkable that the GPS receiver is installed inside the navigation system.
However, the dielectric antenna is easy to be influenced with a metal disposed close to the dielectric antenna and a condition of a ground because of a function of the dielectric. If the dielectric antenna is influenced with those, a gain of the dielectric antenna is reduced. In such a situation, for example, in the keyless entry system, the receiver cannot receive the signals from the transmitter in a certain direction, so that the keyless entry system has a blind area. In the navigation system, it cannot calculate the position of the vehicle because the GPS receiver cannot receive GPS signals in a certain direction. Therefore, the dielectric antenna is useless for the receiver of the vehicle.